hiya
you're right in that 20% CAT will be payable on anything over €52,108 (in 2008) to your sister - on the assumption that she hasn't received any gift or inheritance from anyone else within her group (b) threshold (ie brother,sister,uncle,aunt...) since 1991. you are also correct in that on top of the €52,108 you can gift her €3,000 every year using the small gifts exemption. this is a very useful tool. maybe you could look into giving her the €52,108 now and give her 3k every year from now on. if you give her an interest free loan then there will be gift tax on the interest free element of the loan (it is classed the same as "free use of property" under the CAT act). if you give it to your mother, to give to your sister, then revenue will look through it and deem that it came from you.
hope that helps